Process for the manufacture of fluoroelastomers having bromine or lodine atom cure sites

ABSTRACT

Fluoroelastomers having bromine, iodine or both iodine and bromine cure sites are prepared by an emulsion polymerization process wherein any iodine or bromine containing comonomers and any iodine or bromine containing chain transfer agents are introduced to the reactor as aqueous emulsions, optionally containing a surfactant.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/731,381 filed Oct. 28, 2005.

FIELD OF THE INVENTION

This invention relates to a process for the manufacture offluoroelastomers having bromine, iodine or both bromine and iodine atomcure sites wherein the comonomer or chain transfer agent containing saidbromine or iodine atoms is added to the polymerization reactor in theform of an aqueous emulsion.

BACKGROUND OF THE INVENTION

Fluoroelastomers having excellent heat resistance, oil resistance, andchemical resistance have been widely employed for sealing materials,containers, and hoses. Examples of fluoroelastomers include copolymerscomprising units of vinylidene fluoride (VF₂) and units of at least oneother fluorine-containing monomer such as hexafluoropropylene (HFP),tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinylfluoride (VF) and fluoroethers such as a perfluoro(alkyl vinyl ether)(PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether)(PMVE), perfluoro(ethyl vinyl ether), and perfluoro(propyl vinyl ether).Other examples of fluoroelastomers include copolymers oftetrafluoroethylene with a perfluoro(alkyl vinyl ether) such asperfluoro(methyl vinyl ether). Ethylene (E) and propylene (P) arenon-fluorinated monomers that are also often used to preparefluoroelastomers.

In order to develop the physical properties necessary for most end useapplications, fluoroelastomers must be crosslinked. A preferred curingsystem for many end uses is the combination of an organic peroxide and amultifunctional unsaturated coagent. The coagent forms crosslinks byreacting with cure sites on the backbone of the fluoroelastomer polymerchain. A preferred cure site is a bromine or an iodine atom bonded to acarbon atom on the fluoroelastomer chain.

Fluoroelastomers are typically prepared by free radical emulsionpolymerization. One method of introducing iodine or bromine cure sitesinto the fluoroelastomer is by conducting the polymerization in the pppresence of a chain transfer agent containing iodine or bromine. In thismanner an iodine or bromine atom is attached to the resultingfluoroelastomer at one or more terminal points. Such chain transferagents generally have the structure RX_(n), where R may be a C₁-C₃hydrocarbon, a C₁-C₆ fluorohydrocarbon, a C₁-C₆ chlorofluorohydrocarbonor a C₂-C₈ perfluorocarbon, X is iodine or bromine, and n=1 or 2 (U.S.Pat. Nos. 4,243,770 and 4,973,633).

Another common method of introducing iodine or bromine cure sites onto afluoroelastomer polymer chain is by copolymerizing a minor amount of aniodine or bromine-containing fluoroolefin or fluorovinyl ether cure sitemonomer with other monomers (e.g. VF₂, HFP, TFE, PAVE, P, E, etc.). Inthis manner, cure sites may be randomly distributed along the resultingpolymer chain.

A general problem with the use of either iodine- or bromine-containingcure sites or comonomers is related to the generally high specificgravity of these compounds. For example, at 25° C., the specific gravityof methylene iodide (CH₂I₂) is 3.33, the specific gravity of methylenebromide (CH₂Br₂) is 2.51, the specific gravity of1,4-diiodooctafluorobutane is 2.48, and the specific gravity of1,6-diiodododecafluorohexane is 2.35. When materials of such highspecific gravity are added to a polymerization reactor, they tend tosettle to the bottom of the reactor. Because these materials settle tothe reactor bottom, they may be incorporated unevenly into polymerchains such that a portion of the chains has an abnormally high amountof cure site while other portions have an abnormally low amount of curesite. This will result in undesirable variability in the end useperformance of products made from fluoroelastomers. Another consequenceof the high specific gravity is that a portion of the iodine- orbromine-containing compound may not even become incorporated into thepolymer, resulting in reduced efficiency of the iodine- orbromine-containing compound, and the need for additional waste treatmentfacilities to capture any residual iodine- or bromine-containing curesite or chain transfer agent. Yet another consequence of the highspecific gravity is poor viscosity control of the resultingfluoroelastomer polymer, because the iodine- or bromine-containing chaintransfer agent does not become evenly incorporated into the polymerchains.

Another problem with the use of many iodine- or bromine-containing curesite monomers or chain transfer agents is their lack of solubility inwater. This low solubility can cause variable incorporation of thesecompounds in emulsion polymerization processes (J. Appl. Polym. Sci. 51,21 (1994)). In order to achieve complete incorporation of a poorlysoluble chain transfer agent into a polymer, the overall polymerizationrate may need to be decreased, which leads to an inefficient use ofpolymerization reactors.

To solve the problems of high specific gravity and low aqueoussolubility, it has been proposed to dissolve the iodine- orbromine-containing cure site monomer or chain transfer agent in anorganic solvent and inject this solution into the polymerization reactor(U.S. Pat. Nos. 4,973,633 and 5,284,920). However, this approach isunsatisfactory because it requires additional waste treatment facilitiesto remove the solvent from either the polymer or the waste water. Inaddition, it is difficult to find satisfactory solvents that do notinterfere with the polymerization reaction and slow it down.

Another proposed solution is to incorporate an iodine-containing chaintransfer agent into a spontaneously generated fluorinated microemulsion(U.S. Pat. No. 5,585,449). However, this method has the drawback thatthe fluorinated oils that comprise the microemulsion are retained in thepolymer. These can be detected (e.g. by headspace GC-MS), and mayadversely affect adhesion to metals and mill behavior, as well as foodcontact status.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a process for themanufacture of fluoroelastomers comprising the use of amechanically-generated emulsion of an iodine- or bromine-containingchain transfer agent and/or an iodine- or bromine-containing cure sitemonomer in which the average droplet size of the resulting emulsion isless than 50 microns, and which is substantially free of any organicsolvent or oil, and which may optionally contain a surfactant.

In another aspect, the present invention provides a process forpreparing a fluoroelastomer having bromine, iodine or both bromine andiodine cure sites. The process comprises:

(A) charging a reactor with a quantity of an aqueous solution;

(B) feeding to said reactor a quantity of an initial monomer mixture toform a reaction medium, said initial monomer mixture comprising i) afirst monomer, said first monomer selected from the group consisting ofvinylidene fluoride and tetrafluoroethylene, and ii) one or moreadditional copolymerizable monomers, different from said first monomer,wherein said additional monomer is selected from the group consisting offluorine-containing olefins, fluorine-containing ethers, propylene,ethylene and mixtures thereof;

(C) feeding to said reactor at least one aqueous emulsion comprising acure site source selected from the group consisting of i) aniodine-containing cure site monomer, ii) a bromine-containing cure sitemonomer, iii) an iodine-containing chain transfer agent, and iv) abromine-containing chain transfer agent; wherein said emulsion has adroplet size of less than 50 microns; and

(D) polymerizing said monomers in the presence of a free radicalinitiator to form a fluoroelastomer having cure sites.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an emulsion polymerization processfor manufacturing fluoroelastomers that contain bromine, iodine or bothbromine and iodine atom cure sites.

The fluoroelastomers prepared by the process of this invention comprisecopolymerized units of a first monomer which may be vinylidene fluoride(VF₂) or tetrafluoroethylene (TFE) and one or more additional monomers,different from said first monomer, selected from the group consisting offluorine-containing olefins, fluorine-containing ethers, propylene,ethylene and mixtures thereof. The level of copolymerized units of firstmonomer present in fluoroelastomers prepared by the process of thisinvention is no more than 85 mole percent, based on total number ofmoles of all copolymerized monomers incorporated in the fluoroelastomer.

Examples of fluorine-containing olefins copolymerizable with the firstmonomer include vinylidene fluoride, hexafluoropropylene (HFP),tetrafluoroethylene (TFE), 1,2,3,3,03-pentafluoropropene (1-H PFP),chlorotrifluoroethylene (CTFE) and vinyl fluoride.

Examples of fluorine-containing ethers that may be employed in thepresent invention include perfluoro(alkyl vinyl ethers), perfluoro(alkylalkenyl ethers) and perfluoro(alkoxy alkenylethers).

Perfluoro(alkyl vinyl ethers) (PAVE) suitable for use as monomersinclude those of the formulaCF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)  (I)where R_(f′) and R_(f″)are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl ethers) includes compositionsof the formulaCF₂═CFO(CF₂CFXO)_(n)R_(f)  (II)

-   -   where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl        group of 1-6 carbon atoms.        A most preferred class of perfluoro(alkyl vinyl ethers) includes        those ethers wherein n is 0 or 1 and R_(f) contains 1-3 carbon        atoms. Examples of such perfluorinated ethers include        perfluoro(methyl vinyl ether) (PMVE) and perfluoro(propyl vinyl        ether) (PPVE). Other useful monomers include compounds of the        formula        CF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)  (III)        where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms,        m=0 or 1, n=0-5, and Z=F or CF₃.        Preferred members of this class are those in which R_(f) is        C₃F₇, m=0, and n=1.

Additional perfluoro(alkyl vinyl ether) monomers include compounds ofthe formulaCF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (IV)

-   -   where m and n independently=0-10, p=0-3, and x=1-5.        Preferred members of this class include compounds where n=0-1,        m=0-1, and x=1.

Other examples of useful perfluoro(alkyl vinyl ethers) includeCF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)  (V)where n=1-5, m=1-3, and where, preferably, n=1.

Perfluoro(alkyl alkenyl ethers) suitable for use as monomers includethose of the formula VIR_(f)O(CF₂)_(n)CF═CF₂  (VI)where R_(f) is a perfluorinated linear or branched aliphatic groupcontaining 1-20, preferably 1-10, and most preferably 1-4 carbon atomsand n is an integer between 1 and 4. Specific examples includeperfluoro(propoxyallyl ether) and perfluoro(propoxybutenyl ether).

Perfluoro(alkoxy alkenyl ethers) differ from perfluoro(alkyl alkenylethers) in that R_(f) in formula VI contains at least one oxygen atom inthe aliphatic chain. A specific example includes, but is not limited toperfluoro(methoxyethoxyallyl ether).

If copolymerized units of a fluorine-containing ether are present in thefluoroelastomers of the invention, the ether unit content generallyranges from 25 to 75 weight percent, based on the total weight of thefluoroelastomer. If perfluoro(methyl vinyl) ether is used, then thefluoroelastomer preferably contains between 30 and 55 wt. %copolymerized PMVE units.

Fluoroelastomers prepared by the process of this invention also containcure sites suitable for organic peroxide induced crosslinking. Thesource of the cure sites may be i) a copolymerizable cure site monomercontaining bromine or iodine, ii) a bromine or iodine-containing chaintransfer agent, or iii) both i) and ii). The level of bromine or iodineatoms incorporated into the fluoroelastomers is between 0.03 and 1.5mole percent, based on the total number of moles of copolymerizedmonomers in the fluoroelastomers. If the fluoroelastomer contains bothiodine- or bromine-containing cure site monomers and iodine- orbromine-containing endgroups (resulting from chain transfer agents), thelevel of iodine or bromine atoms may be in the range of 0.03 to 1.5 molepercent from each of the cure site sources (i.e. from the cure sitemonomer and from the chain transfer agent), for a total of 0.06 to 3mole percent iodine or bromine cure sites.

Bromine atom containing cure site monomers may contain other halogens,preferably fluorine. Examples of brominated olefin cure site monomersare bromotrifluoroethylene; 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB);and others such as vinyl bromide, 1- bromo-2,2-difluoroethylene;perfluoroallyl bromide; 4-bromo-1,1,2-trifluorobutene-1;4-bromo-1,1,3,3,4,4,-hexafluorobutene; 4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene; 6-bromo-5,5,6,6-tetrafluorohexene;4-bromoperfluorobutene-1 and 3,3-difluoroallyl bromide. Brominated vinylether cure site monomers useful in the invention include2-bromo-perfluoroethyl perfluorovinyl ether and fluorinated compounds ofthe class CF₂Br—R_(f)—O—CF═CF₂ (R_(f) is a perfluoroalkylene group),such as CF₂═CFOCF₂CF₂CF₂OCF₂CF₂Br; CF₂BrCF₂O—CF═CF₂, and fluorovinylethers of the class ROCF═CFBr or ROCBr═CF₂ (where R is a lower alkylgroup or fluoroalkyl group) such as CH₃OCF═CFBr or CF₃CH₂OCF═CFBr.

Suitable iodine atom containing cure site monomers include iodinatedolefins of the formula: CHR═CH-Z-CH₂CHR—I, wherein R is —H or —CH₃; Z isa C₁-C₁₈ (per)fluoroalkylene radical, linear or branched, optionallycontaining one or more ether oxygen atoms, or a(per)fluoropolyoxyalkylene radical as disclosed in U.S. Pat. No.5,674,959. Other examples of useful iodinated cure site monomers areunsaturated ethers of the formula: I(CH₂CF₂CF₂)_(n)OCF═CF₂ andICH₂CF₂O[CF(CF₃)CF₂O]_(n)CF═CF₂, and the like, wherein n=1-3, such asdisclosed in U.S. Pat. No. 5,717,036. In addition, suitable iodinatedcure site monomers including iodoethylene,4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB);3-chloro4-iodo-3,4,4-trifluorobutene;2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane;2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene;1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethylvinyl ether; 3,3,4,5,5,5-hexafluoro4-iodopentene; andiodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyliodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also usefulcure site monomers.

If a bromine or iodine containing cure site monomer is employed in theprocess of the invention, it is introduced to the reactor in the form ofan aqueous emulsion (described hereinafter).

In addition to, or instead of a cure site monomer, iodine or bromineatom containing endgroups may optionally be present at one or both ofthe fluoroelastomer polymer chain ends as a result of the use of bromineor iodine atom containing chain transfer or molecular weight regulatingagents during preparation of the fluoroelastomers. The chain transferagent is typically of the formula RX_(n), where R may be a C₁-C₃hydrocarbon, a C₁-C₆ fluorohydrocarbon, a C₁-C₆ chlorofluorohydrocarbonor a C₂-C₈ perfluorocarbon, X is iodine or bromine, and n=1 or 2 (U.S.Pat. Nos. 3,707,529 and 4,243,770). Such agents include those of formulaCH₂X₂ where X is I or Br; X(CF₂)_(n)Y where X is I or Br, Y is I or Br(preferably both X and Y are I) and n is an integer between 3 and 10.

Specific examples include methylene iodide; 1,3-diiodoperfluoropropane;1,4-diiodoperfluorobutane; 1,6-diiodoperfluorohexane;1,8-diiodoperfluorooctane; 1,10-diiodoperfluorodecane; and1-iodo-nonafluorobutane. Other chain transfer agents such as those offormula RBr_(n)I_(m) (R is as defined above; n and m each are 1 or 2)may also be used. Particularly preferred are diiodinated perfluoroalkanechain transfer agents and mixtures thereof.

If a bromine or iodine containing chain transfer agent is employed inthe process of the invention, it is introduced to the reactor in theform of an aqueous emulsion. If both a cure site monomer and a chaintransfer agent are employed for introducing cure sites onto thefluoroelastomer, the cure site monomer and chain transfer agent aretypically in separate aqueous emulsions that can be added to the reactorat different times and at different rates.

Aqueous emulsions of cure site monomer or chain transfer agent aretypically prepared by high shear mechanical mixing. An aqueous phase,which optionally contains a surfactant, is contacted with an organicphase, which contains either a cure site monomer or a chain transferagent or a mixture of both, in the presence of a device that generates ahigh shear field. The shear field may be generated by a device withmoving or rotating parts such as a homogenizer or rotor/statorcombination. Alternatively, the high shear field may be generated bydevices with essentially no moving parts such as a static mixer ormicromixer. Emulsions may be prepared separately away from the reactor,temporarily stored until needed, and then transferred into thepolymerization reactor. Alternatively, the emulsion may be preparedin-line whereby the aqueous phase and the organic phase aresimultaneously fed into a device that creates the emulsion andimmediately transfers it into the reactor with essentially no temporarystorage. The resulting aqueous emulsion has a mean droplet size of 50microns or less, preferably 20 microns or less.

Optionally a surfactant may be employed to help in stabilizing theemulsions. Specific examples of suitable surfactants include alkylsulfonates such as sodium octyl sulfonate and sodium dodecylsulfonate,alkyl sulfates such as sodium lauryl sulfate and sodium decyl sulfate,alkyl carboxylates such as sodium caprylate and sodium stearate,nonionic surfactants such as nonylphenolpoly(ethylene oxide) andalkylpoly(ethylene oxide), perfluorinated carboxylic acids such asperfluorohexylethylsulfonic acid, perfluorooctanoic acid and theirsalts, partially fluorinated sulfonic acids such astridecafluorohexylethyl sulfonic acid and its salts, and partiallyfluorinated carboxylic acids such as3,3,4,4-tetrahydroundecafluorooctanoic acid and its salts.

Surfactant, if present, is typically at the level of 0.05 to 5% byweight in the aqueous phase. The amount of surfactant used in theemulsion will depend on the specific requirements of the process andproduct. Generally, higher levels of surfactant increase the stabilityof the emulsion, but introduce more potential impurities into the finalfluoroelastomer that may be deleterious to end use performance. Ifinsufficient surfactant is used, the resulting emulsion will displayinsufficient stability as evidenced by droplet coalescence and formationof a separate organic phase that is visible to the naked eye. When theemulsion is temporarily stored, by e.g. placing in a storage tank,longer emulsion stability is required than if the emulsion is feddirectly into the polymerization reactor. Once a separate organic phaseis created, the benefits conferred by this invention are lost.

The emulsion polymerization process of this invention may be operatedeither in semi-batch or continuous fashion. In a semi-batch process, agaseous monomer mixture of a desired composition (initial monomercharge) is introduced into a reactor which contains an aqueous solution.The aqueous solution may optionally comprise a surfactant emulsifyingagent such as a fluorosurfactant (e.g. ammonium perfluorooctanoate,Zonyl® FS-62 (available from DuPont) or Foraface® 1033D (available fromDuPont)), or a hydrocarbon surfactant (e.g. sodium dodecyl sulfonate).Optionally, the aqueous solution may also contain inorganic salts suchas a pH buffer (e.g. a phosphate or acetate buffer for controlling thepH of the polymerization reaction). Instead of a buffer, a base, such asNaOH may be used to control pH. Generally, pH is controlled to between 1and 10 (preferably 3-7), depending upon the type of fluoroelastomerbeing made. Alternatively, or additionally, pH buffer or base may beadded to the reactor at various times throughout the polymerizationreaction, either alone or in combination with other ingredients such aspolymerization initiator, an aqueous emulsion of liquid cure sitemonomer or an aqueous emulsion of chain transfer agent. Also optionally,the initial aqueous solution may contain a water-soluble inorganicperoxide polymerization initiator such as ammonium persulfate (or otherpersulfate salt), or the combination of an inorganic peroxide and areducing agent such as the combination of ammonium persulfate and sodiumsulfite.

The initial monomer charge contains a quantity of a first monomer ofeither TFE or VF₂ and one or more additional monomers which aredifferent from the first monomer. The amount of monomer mixturecontained in the initial charge is set so as to result in a reactorpressure between 0.5 and 10 MPa (preferably between 0.5 and 3.5 MPa). Inthe initial gaseous monomer charge, the relative amount of each monomeris dictated by reaction kinetics and is set so as to result in afluoroelastomer having the desired ratio of copolymerized monomer units(i.e. very slow reacting monomers must be present in a higher amountrelative to the other monomers than is desired in the composition of thefluoroelastomer to be produced).

The monomer mixture is dispersed in the aqueous medium and, optionally,an aqueous emulsion of chain transfer agent may also be introduced atthis point while the reaction mixture is agitated, typically bymechanical stirring. Alternatively, if employed in the process of theinvention, the aqueous emulsion of chain transfer agent may beintroduced at any time up to the point when all of the incrementalmonomer mixture has been fed to the reactor. The entire amount of chaintransfer agent may be added at one time, or addition may be spread outover time, up to the point when 100% of the incremental monomer mixturehas been added to the reactor. Most preferably, the chain transfer agentaqueous emulsion is introduced to the reactor before polymerizationbegins, or shortly thereafter, and the entire amount of chain transferagent is fed to the reactor by the time that 5 wt. % of the total amountof incremental monomer mixture has been fed to the reactor.

The temperature of the semi-batch reaction mixture is maintained in therange of 25° C.-130° C., preferably 30° C.-90° C. Polymerization beginswhen the initiator either thermally decomposes or reacts with reducingagent and the resulting radicals react with dispersed monomer.

Additional quantities of the gaseous monomers (referred to herein asincremental monomer mixture feed) are added at a controlled ratethroughout the polymerization in order to maintain a constant reactorpressure at a controlled temperature. The relative ratio of gaseousmonomers contained in the incremental monomer mixture feed is set to beapproximately the same as the desired ratio of copolymerized monomerunits in the resulting fluoroelastomer. If employed in the process ofthe invention, additional chain transfer agent aqueous emulsion mayalso, optionally, be continued to be added to the reactor at any pointduring this stage of the polymerization. Additional surfactant andpolymerization initiator may also be fed to the reactor during thisstage. The amount of polymer formed is approximately equal to thecumulative amount of incremental monomer mixture feed. One skilled inthe art will recognize that the molar ratio of monomers in theincremental gaseous monomer mixture feed is not necessarily exactly thesame as that of the desired copolymerized monomer unit composition inthe resulting fluoroelastomer because the composition of the initialcharge may not be exactly that required for the desired finalfluoroelastomer composition, or because a portion of the monomers in theincremental monomer mixture feed may dissolve into the polymer particlesalready formed, without reacting.

If a copolymerizable cure site monomer is employed in the process of theinvention, a stream of cure site monomer aqueous emulsion is fed to thereactor at a rate so as to result in the entire amount of cure sitemonomer emulsion being fed to the reactor by the time that 99 wt. % ofthe incremental monomer mixture has been fed. Total polymerization timesin the range of from 2 to 30 hours are typically employed in thissemi-batch polymerization process.

The continuous emulsion polymerization process of this invention differsfrom the semi-batch process in the following manner. The reactor iscompletely filled with aqueous solution so that there is no vapor space.Gaseous monomers and solutions of other ingredients such aswater-soluble monomers, aqueous emulsions of chain transfer agents,buffer, bases, polymerization initiator, surfactant, etc., are fed tothe reactor in separate streams at a constant rate. Feed rates arecontrolled so that the average polymer residence time in the reactor isgenerally between 0.2 to 4 hours. Short residence times are employed forreactive monomers, whereas less reactive monomers such asperfluoro(alkyl vinyl) ethers require more time. The temperature of thecontinuous process reaction mixture is maintained in the range of 25°C.-130° C., preferably 80° C.-120° C.

In the process of this invention, the polymerization temperature ismaintained in the range of 25°-130° C. If the temperature is below 25°C., the rate of polymerization is too slow for efficient reaction on acommercial scale, while if the temperature is above 130° C., the reactorpressure required in order to maintain polymerization is too high to bepractical.

The polymerization pressure is controlled in the range of 0.5 to 10 MPa,preferably 1 to 6.2 MPa. In a semi-batch process, the desiredpolymerization pressure is initially achieved by adjusting the amount ofgaseous monomers in the initial charge, and after the reaction isinitiated, the pressure is adjusted by controlling the incrementalgaseous monomer feed. In a continuous process, pressure is adjusted bymeans of a back-pressure regulator in the dispersion effluent line. Thepolymerization pressure is set in the above range because if it is below1 MPa, the monomer concentration in the polymerization reaction systemis too low to obtain a satisfactory reaction rate. In addition, themolecular weight does not increase sufficiently. If the pressure isabove 10 MPa, the cost of the required high pressure equipment is veryhigh.

The amount of fluoroelastomer copolymer formed is approximately equal tothe amount of incremental feed charged, and is in the range of 10-30parts by weight of copolymer per 100 parts by weight of aqueous medium,preferably in the range of 20-25 parts by weight of the copolymer. Thedegree of copolymer formation is set in the above range because if it isless than 10 parts by weight, productivity is undesirably low, while ifit is above 30 parts by weight, the solids content becomes too high forsatisfactory stirring.

Water-soluble peroxides which may be used to initiate polymerization inthis invention include, for example, the ammonium, sodium or potassiumsalts of hydrogen persulfate. In a redox-type initiation, a reducingagent such as sodium sulfite, is present in addition to the peroxide.These water-soluble peroxides may be used alone or as a mixture of twoor more types. The amount to be used is selected generally in the rangeof 0.01 to 0.4 parts by weight per 100 parts by weight of polymer,preferably 0.05 to 0.3. During polymerization some of thefluoroelastomer polymer chain ends are capped with fragments generatedby the decomposition of these peroxides.

The resulting fluoroelastomer emulsion, prepared by either semi-batch orcontinuous processes, may be isolated, filtered, washed and dried byconventional techniques employed in the fluoroelastomer manufacturingindustry.

In addition to cure site, preferred fluoroelastomers of this inventioncomprise copolymerized units of i) vinylidene fluoride andhexafluoropropylene; ii) vinylidene fluoride, hexafluoropropylene andtetrafluoroethylene; iii) vinylidene fluoride, tetrafluoroethylene andperfluoro(methyl vinyl ether); and iv) tetrafluoroethylene andperfluoro(methyl vinyl ether).

Fluoroelastomers prepared by the process of this invention can becrosslinked (i.e. vulcanized or cured) by an organic peroxide. Curablefluoroelastomer compositions comprise a) a fluoroelastomer prepared bythe process of this invention (as defined above), b) an organicperoxide, and c) a coagent. Preferably, the compositions also contain anacid acceptor such as a divalent metal hydroxide, a divalent metaloxide, a strongly basic (i.e. pKa>10) organic amine such asProtonSponge® (available from Aldrich), or a combination of any of thelatter. Examples of divalent metal oxides and hydroxides include CaO,Ca(OH)₂ and MgO.

Organic peroxides suitable for use include1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane;1,1-bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane;n-butyl4, 4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane;2,5-dimethylhexane-2,5-dihydroxyperoxide; di-t-butyl peroxide;t-butylcumyl peroxide; dicumyl peroxide; alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene;2,5-dimethyl-2,5-di(t-butylperoxy)hexane;2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide,t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane;t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferredexamples of organic peroxides include2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene. The amount compounded isgenerally in the range of 0.05-5 parts by weight, preferably in therange of 0.1-3 parts by weight per 100 parts by weight of thefluoroelastomer. This particular range is selected because if theperoxide is present in an amount of less than 0.05 parts by weight, thevulcanization rate is insufficient and causes poor mold release. On theother hand, if the peroxide is present in amounts of greater than 5parts by weight, the compression set of the cured polymer becomesunacceptably high. In addition, the organic peroxides may be used singlyor in combinations of two or more types.

Coagents employed in the curable compositions are polyfunctionalunsaturated compounds such as triallyl cyanurate, trimethacrylisocyanurate, triallyl isocyanurate, trimethallyl isocyanurate, triacrylformal, triallyl trimellitate, N,N′-m-phenylene bismaleimide, diallylphthalate, tetraallylterephthalamide, tri(diallylamine)-s-triazine,triallyl phosphite, bis-olefins and N,N-diallylacrylamide. The amountcompounded is generally in the range of 0.1-10 parts by weight per 100parts by weight of the fluoroelastomer. This particular concentrationrange is selected because if the coagent is present in amounts less than0.1 part by weight, crosslink density of the cured polymer isunacceptable. On the other hand, if the coagent is present in amountsabove 10 parts by weight, it blooms to the surface during molding,resulting in poor mold release characteristics. The preferable range ofcoagent is 0.2-6 parts by weight per 100 parts fluoroelastomer. Theunsaturated compounds may be used singly or as a combination of two ormore types.

Optionally, other components, for example fillers such as carbon black,Austin black, graphite, thermoplastic fluoropolymer micropowders,silica, clay, diatomaceous earth, talc, wollastonite, calcium carbonate,calcium silicate, calcium fluoride, and barium sulfate; processing aidessuch as higher fatty acid esters, fatty acid calcium salts, fattyacidamides (e.g. erucamide), low molecular weight polyethylene, siliconeoil, silicone grease, stearic acid, sodium stearate, calcium stearate,magnesium stearate, aluminum stearate, and zinc stearate; coloringagents such as titanium white and iron red may be employed ascompounding additives in compositions containing fluoroelastomersprepared by the process of this invention. The amount of such filler isgenerally in the range of 0.1-100 parts by weight, preferably 1-60 partsby weight, per 100 parts by weight of the fluoroelastomer. This range isselected because if the filler is present in amounts of less than 0.1part by weight, there is little or no effect, while, on the other hand,if greater than 100 parts by weight are used, elasticity is sacrificed.The amount of processing aid compounded is generally less than 10 partsby weight, preferably less than 5 parts by weight, per 100 parts byweight of the fluoroelastomer. If the amount used is above the limit,heat resistance is adversely affected. The amount of a coloring agentcompounded is generally less than 50 parts by weight, preferably lessthan 30 parts by weight per 100 parts by weight of the fluoroelastomer.If greater than 50 parts by weight is used, compression set suffers.

The fluoroelastomer, organic peroxide, coagent, and any otheringredients are generally incorporated into curable compositions bymeans of an internal mixer or rubber mill. The resulting composition maythen be shaped (e.g. molded or extruded) and cured. Curing typicallytakes place at about 150°-200° C. for 1 to 60 minutes. Conventionalrubber curing presses, molds, extruders, and the like provided withsuitable heating and curing means can be used. Also, for optimumphysical properties and dimensional stability, it is preferred to carryout a post curing operation wherein the molded or extruded article isheated in an oven or the like for an additional period of about 14-8hours, typically from about 180°-275° C., generally in an airatmosphere.

The fluoroelastomers prepared by the process of this invention areuseful in many industrial applications including seals, wire coatings,tubing and laminates.

EXAMPLES Test Methods

Mooney viscosity, ML (1+10), was determined according to ASTM D1646 withan L (large) type rotor at 121° C. (unless otherwise noted), using apreheating time of one minute and rotor operation time of 10 minutes.

Inherent viscosities were measured at 30° C. Methyl ethyl ketone wasemployed as solvent (0.1 g polymer in 100 ml solvent) forfluoroelastomers that contained copolymerized units of vinylidenefluoride. A mixed solvent of 60/40/3 volume ratio ofheptafluoro-2,2,3-trichlorobutane, perfluoro(α-butyltetrahydrofuran) andethylene glycol dimethyl ether was used (0.2 g polymer in 100 mlsolvent) for fluoroelastomers containing copolymerized units oftetrafluoroethylene and perfluoro(methyl vinyl ether).

Iodine content of the polymers was measured by X-ray fluorescenceanalysis of the isolated, dried polymer.

Emulsion droplet size was measured at room temperature with a Coulter LSParticle Size Analyzer and a 61 second analysis time.

Emulsion Preparation

Two different methods for preparation of perfluoroalkyl diiodide aqueousemulsions were used in the following examples. These preparation methodsshould not be considered limiting. Other methods for preparing emulsionsare known to those skilled in the art. In Method A, a mixture of1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane, with a flowrate of 1 milliliter per minute, and a 1 wt. %perfluorohexylethylsulfonic acid solution in water, with a flow rate of10 milliliters per minute, were simultaneously passed through a SIMM-LASmicromixer (manufactured by IMM, Mainz, Germany) to form an emulsionwith a mean droplet size of 5.7 microns and with 95% of all dropletsless than 15 microns.

In Method B, 22.5 milliliters of a mixture of 1,4-diiodooctafluorobutaneand 1,6-diiodododecafluorohexane were added together with 427.5milliliters of a 1 wt. % perfluorohexylethylsulfonic acid solution inwater into a Microfluidics M-110Y microfluidizer. This mixture waspassed through the microfluidizer 4 times to generate a 5 volume percentemulsion of the diiodo compounds that had a mean droplet size of 0.18microns and with 95% of all droplets less than 0.27 microns.

Example 1

A 41 liter reactor was charged with a water solution containing 17.5grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphateheptahydrate, and 24,969.6 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride,3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether).30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. %disodium phosphate heptahydrate was added to initiate polymerization. Asthe reactor pressure dropped, a monomer feed of 55 wt. % vinylidenefluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perperfluoro(methyl vinyl ether) was added to maintain pressure. After 90grams of this monomer mixture had been added, an emulsion of a mixtureof 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in a 1%(wt basis) perfluorohexylethylsulfonic acid solution in water, preparedaccording to Method A (above), was fed into the reactor. After a totalof 30.0 grams of the diiodide mixture had been fed to the reactor, thediiodide mixture feed was discontinued and the aqueous 1 wt. %perfluorohexylethylsulfonic solution fed for another minute before alsobeing shut off. Additional initiator solution was added as needed tomaintain polymerization. After a total of 8,333 grams of the mixture of55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. %per perfluoro(methyl vinyl ether) had been fed to the reactor, thereaction was stopped and the reactor depressurized. 33,525 grams of a23.76 wt. % solids latex was obtained. The polymer was isolated byadding aluminum sulfate to the latex, and then dried at 70° C.

Example 2

A 41 liter reactor was charged with a water solution containing 17.5grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphateheptahydrate, and 24,969.6 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride,3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether).30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. %disodium phosphate heptahydrate was added to initiate polymerization. Asthe reactor pressure dropped, a monomer feed of 55 wt. % vinylidenefluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perperfluoro(methyl vinyl ether) was added to maintain pressure. After 90grams of this monomer mixture had been added, an emulsion of a mixtureof 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1% (wtbasis) perfluorohexylethylsulfonic acid solution in water, preparedaccording to Method A (above) was then fed into the reactor. After atotal of 10.0 grams of the diiodide mixture had been fed to the reactor,the diiodide mixture feed was discontinued and the aqueous 1 wt. %perfluorohexylethylsulfonic solution fed for another minute before alsobeing shut off. After 833 grams of monomer mixture had been added, anemulsion of a mixture of 1,4-diiodooctafluorobutane and1,6-diiodododecafluorohexane in 1% (wt basis)perfluorohexylethylsulfonic acid solution in water, again preparedaccording to Method A, was fed to the reactor. After an additional 20.0grams of the diiodide mixture had been fed to the reactor, the diiodidemixture feed was discontinued and the aqueous 1 wt. %perfluorohexylethylsulfonic solution fed for another minute before alsobeing shut off. Additional initiator solution was added, as needed, tomaintain polymerization. After a total of 8,333 grams of the mixture of55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. %per perfluoro(methyl vinyl ether) had been fed to the reactor, thereaction was stopped and the reactor depressurized. 33,800 grams of a25.39 wt. % solids latex was obtained. The polymer was isolated byadding aluminum sulfate to the latex, and then dried at 70° C.

Comparative Example 1

A 41 liter reactor was charged with a water solution containing 17.5grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphateheptahydrate, and 24,969.6 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride,3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether).30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. %disodium phosphate heptahydrate was added to initiate polymerization. Asthe reactor pressure dropped, a monomer feed of 55 wt. % vinylidenefluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perperfluoro(methyl vinyl ether) was added to maintain pressure. After 90grams of this monomer mixture had been added, a total of 30.0 grams of amixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexanewas fed neat to the reactor over a 10 minute period. Additionalinitiator solution was added as needed to maintain polymerization. Aftera total of 8,333 grams of the mixture of 55 wt. % vinylidene fluoride,10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinylether) had been fed to the reaction, the reaction was stopped and thereactor depressurized. 33,695 grams of a 25.31 wt. % solids latex wasobtained. The polymer was isolated by adding aluminum sulfate to thelatex, and then dried at 70° C.

Analytical results from these examples are shown in Table I. Each ofthese examples used 30.0 grams of the mixture of1,4-diiodooctafluorobutane and 1,6-diiodo dodecafluorohexane. The iodinecontent of this mixture was 48.6 wt. %. Therefore, each example polymerreceived 14.58 grams iodine. TABLE I Example 1 2 Comparative 1 Gramspolymer 7965 8602     8528 Inherent Viscosity 0.71 0.72 0.94 Mooneyviscosity 41.2 43.1  79.3 Mol % Iodine 0.12 0.12 0.09 Wt. % Iodine 0.182 0.177 0.140 Grams I in polymer 14.50 15.22  11.93 Iodine yield, % 99104¹    82¹within experimental error 100%

The data in Table I show that the use of perfluoroalkyldiiodide emulsionincreased iodine incorporation into the polymer to essentially 100% andthat the resulting polymer had lower inherent and Mooney viscositiesthan did a comparative example made with neat perfluoroalkyldiiodide.

Example 3

A 41 liter reactor was charged with a water solution containing 17.5grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphateheptahydrate, and 24,969.6 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride,3 wt. % tetrafluoroethylene, and 54 wt % perfluoro(methyl vinyl ether).30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. %disodium phosphate heptahydrate was added to initiate polymerization. Asthe reactor pressure dropped, a monomer feed of 55 wt. % vinylidenefluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perfluoro(methylvinyl ether) was added to maintain pressure. After 90 grams of thismixture had been added, a 5 volume % emulsion of a mixture of1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1 wt. %perfluorohexylethylsulfonic solution, prepared as described in Method B(above), was fed at the rate of 25 mL/minute. After 10 minutes this feedwas stopped. Additional initiator solution was added as needed tomaintain polymerization. After a total of 8,333 grams of the mixture of55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. %perfluoro(methyl vinyl ether) had been fed to the reaction, the reactionwas stopped and the reactor depressurized. A 24.95 wt. % solids latexwas obtained. The polymer was isolated by adding aluminum sulfate to thelatex, and then dried at 70° C. The polymer had a Mooney viscosity of 42and an inherent viscosity of 0.72.

Example 4

A 41 liter reactor was charged with a water solution containing 34.5grams perfluorohexylethylsulfonic acid, 40.0 grams disodium phosphateheptahydrate, and 24,925.5 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 2.00 MPag with a mixture of 25 wt. % tetrafluoroethylene,and 75 wt % perfluoro(methyl vinyl ether). 40.0 grams of a solution of 1wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydratewas added to initiate polymerization. As the reactor pressure dropped, amonomer feed of 52 wt. % tetrafluoroethylene, and 48 wt. %perfluoro(methyl vinyl ether) was added to maintain pressure. After 45grams of this mixture had been added, a 9 volume% emulsion of a mixtureof 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1 wt.% perfluorohexylethylsulfonic solution, prepared as described in MethodA (above), was fed at the rate of 16.5 mL/minute. After 7 minutes thisfeed was stopped. Additional initiator solution was added as needed tomaintain polymerization. After a total of 8,333 grams of the mixture of52 wt. % tetrafluoroethylene, and 48 wt. % perfluoro(methyl vinyl ether)had been fed to the reaction, the reaction was stopped and the reactordepressurized. A 24.03 wt. % solids latex was obtained. The polymer wasisolated by adding aluminum sulfate to the latex, and then dried at 70°C. The polymer had a Mooney viscosity of 69.5.

Example 5

A 41 liter reactor was charged with a water solution containing 24.7grams perfluorohexylethylsulfonic acid, 20.0 grams disodium phosphateheptahydrate, and 24,955.3 grams deionized water. The reactor wasbrought to 80° C. and flushed with nitrogen to remove oxygen and thenpressurized to 1.72 MPag with a mixture of 25 wt. % vinylidene fluoride,2 wt. % tetrafluoroethylene, and 73 wt % hexafluoropropylene. 50.0 gramsof a solution of 1 wt. % ammonium persulfate and 5 wt. % disodiumphosphate heptahydrate was added to initiate polymerization. As thereactor pressure dropped, a monomer feed of 50 wt. % vinylidenefluoride, 20 wt. % tetrafluoroethylene, and 30 wt. % hexafluoropropylenewas added to maintain pressure. After 45 grams of this mixture had beenadded, a 9 volume % emulsion of a mixture of 1,4-diiodooctafluorobutaneand 1,6-diiodododecafluorohexane in 1 wt. % perfluorohexylethylsulfonicsolution, prepared as described in Method A (above), was fed at the rateof 16.5 muminute. After 12.5 minutes this feed was stopped. Additionalinitiator solution was added as needed to maintain polymerization. Aftera total of 8,333 grams of the mixture of 50 wt. % vinylidene fluoride,20 wt. % tetrafluoroethylene, and 30 wt. % hexafluoropropylene had beenfed to the reaction, the reaction was stopped and the reactordepressurized. A 25.46 wt. % solids latex was obtained. The polymer wasisolated by adding aluminum sulfate to the latex, and then dried at 70°C. The polymer had a Mooney viscosity of 21 and an inherent viscosity of0.55.

1. A process for preparing a fluoroelastomer having bromine, iodine orboth bromine and iodine cure sites, said process comprising: (A)charging a reactor with a quantity of an aqueous solution; (B) feedingto said reactor a quantity of an initial monomer mixture to form areaction medium, said initial monomer mixture comprising i) a firstmonomer, said first monomer selected from the group consisting ofvinylidene fluoride and tetrafluoroethylene, and ii) one or moreadditional copolymerizable monomers, different from said first monomer,wherein said additional monomer is selected from the group consisting offluorine-containing olefins, fluorine-containing ethers, propylene,ethylene and mixtures thereof; (C) feeding to said reactor at least oneaqueous emulsion comprising a cure site source selected from the groupconsisting of i) an iodine-containing cure site monomer, ii) abromine-containing cure site monomer, iii) an iodine-containing chaintransfer agent, and iv) a bromine-containing chain transfer agent;wherein said emulsion has a mean droplet size of 50 microns or less; and(D) polymerizing said monomers in the presence of a free radicalinitiator to form a fluoroelastomer having cure sites.
 2. A process ofclaim 1 wherein said aqueous emulsion in step C) comprises a cure sitesource having a mean droplet size of 20 microns or less.
 3. A process ofclaim 1 wherein said aqueous emulsion in step C) further comprises asurfactant.
 4. A process of claim 3 wherein said surfactant is selectedfrom the group consisting of sodium octyl sulfonate, sodiumdodecylsulfonate, sodium lauryl sulfate, sodium decyl sulfate, sodiumcaprylate, sodium stearate, nonylphenolpoly(ethylene oxide),perfluorohexylethylsulfonic and salts thereof, perfluorooctanoic acidand salts thereof, tridecafluorohexylethyl sulfonic acid and saltsthereof, and 3,3,4,4-tetrahydroundecafluorooctanoic acid and saltsthereof.
 5. A process of claim 3 wherein said aqueous emulsion in stepC) is prepared by high shear mechanical mixing of water, cure sitesource and surfactant.
 6. A process of claim 1 wherein saidfluoroelastomer comprises copolymerized units selected from the groupconsisting of i) vinylidene fluoride and hexafluoropropylene; ii)vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene; iii)vinylidene fluoride, tetrafluoroethylene and perfluoro(methyl vinylether); and iv) tetrafluoroethylene and perfluoro(methyl vinyl ether)and cure sites selected from the group consisting of bromine atoms,iodine atoms and both iodine and bromine atoms.
 7. A process of claim 1wherein said cure site source is a bromine-containing cure site monomerselected from the group consisting of bromotrifluoroethylene;4-bromo-3,3,4,4-tetrafluorobutene-1 vinyl bromide;1-bromo-2,2-difluoroethylene; perfluoroallyl bromide;4-bromo-1,1,2-trifluorobutene-1; 4-bromo-1,1,3,3,4,4,-hexafluorobutene;4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene;6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1;3,3-difluoroallyl bromide; 2-bromo-perfluoroethyl perfluorovinyl ether;CF₂═CFOCF₂CF₂CF₂OCF₂CF₂Br; CH₃OCF═CFBr and CF₃CH₂OCF═CFBr.
 8. A processof claim 1 wherein said cure site source is an iodine-containing curesite monomer selected from the group consisting of iodoethylene;4-iodo-3,3,4,4-tetrafluorobutene-1;3-chloro4-iodo-3,4,4-trifluorobutene;2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane;2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene;1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethylvinyl ether; 3,3,4,5,5,5-hexafluoro4-iodopentene; iodotrifluoroethylene;allyl iodide; and 2-iodo-perfluoroethyl perfluorovinyl ether.
 9. Aprocess of claim 1 wherein said cure site source is an iodine-containingchain transfer agent selected from the group consisting of i) CH₂X₂where X is I or Br; ii) X(CF₂)_(n)Y where X is I or Br, Y is I or Br andn is an integer between 3 and 10 and iii) X(CF₂)_(n)Y where both X and Yare I and n is an integer between 3 and 10.